3:19 Randal talks about ambiguous grammars, which in fact Marpa can handle.
But most new users are going to care about grammars that are unambiguous or nearly so.
Because of its power Marpa can handle, not just any grammar you're likely to want to parse,
but a lot of grammars that you probably did not feel a need to get into.
Consider it like the ability of a good sports car to run smoothly at 120mph.
Not something that's advisable to do on the US highways,
but a sign of the kind of power and handling that can be very nice to have.